Astro Bob blog: Queen of the duskMay begins the season of short nights and languorous dusks.
By: Bob King, Duluth News Tribune
Queen of twilight
Venus draws the eye as you look to the northwestern sky during evening twilight throughout the month of May. The map shows the sky around 9:15 p.m. Created with Stellarium
Venus rules the twilight sky, emerging from daylight blue well before any other star or planet. Look west as early as a half hour after sunset and you'll see it -- a white, flaring disk bigger than any other star in the sky. Venus is more than six times brighter than Jupiter and 40 times the intensity of Saturn. Its relative proximity to Earth combined with a permanent overcast of highly reflective clouds have made it a spectacle since humans looked skyward.
Venus-watchers have a nice twilight glow about them because their planet never strays very far from the sun. Since Venus is one of the two inner planets in the solar system (the other being Mercury), we always look in the general direction of the sun to see it. Unlike Mars and the outer planets, it can't orbit behind the Earth and appear opposite the sun in a dark midnight sky.
I've always enjoyed twilight, especially dusk, when the world seems to slow down and allow time for reflection. One by one the stars appear beginning with the brightest and proceeding by degree until their number is nearly overwhelming. Twilight is defined as the time after sunset and before sunrise when sky and landscape are illuminated indirectly by sunlight reflecting off the atmosphere above. Dusk and dawn are reserved for planets and moons with atmospheres; on the moon and Mercury you have only two choices: sunlight or total darkness with no between-time-twilight to soften the contours of their alien landscapes.
By the middle of May, twilight starts to take a big bite out of night. According to the venerable Old Farmer's Almanac, the time between sunset and the end of astronomical twilight, when the sky is completely dark, grows an additional 11 minutes to 2 hours 13 minutes this week for Duluth and the northern tier of states. The longest twilights come in a month around the summer solstice when we top out at 2 hours 27 minutes.
Venus is a solitary "star" at dusk over Duluth's Canal Park and Aerial Lift Bridge. Photo: Bob King
There are several varieties of twilight. Perhaps you've heard of civil twilight which begins when the center of the sun's disk is six degrees (12 full moon widths) below the horizon. This is when Venus first appears, kids can still play outside and the outlines of houses and trees are sharply defined against the bright, blue-white western sky. Lights come on during nautical twilight when the sun is between six and 12 degrees below the horizon. During this time sailors at sea can still see the horizon and use it as a reference when sighting stars. True night begins only when the sun has dipped to 18 degrees below the horizon at the end of astronomical twilight.
Let's see how much true night is left in my neck of the woods this Saturday when the sky is expected to clear again.
* Sunset Saturday -- 8:37 p.m.
* Evening twilight end -- 10:50 p.m.
* Morning twilight start -- 3:20 a.m.
* Sunrise -- 5:33 a.m.
Night unblemished by any atmospheric glow lasts from 10:50 p.m. until 3:20 a.m. or just 4 1/2 hours. Even I find that hard to believe. Of course for practical purposes the sky is dark enough by 10:30 to see a good number of stars, but for night purists and especially amateur astronomers with telescopes who need real dark to seek the treasures of the deeper sky, we must be patient for all traces of sunlight to vanish.
Depending on where you live and the season, the length of twilight varies. The further north your latitude, the longer twilight's duration in the spring and summer months. If you live north of 48.5 degrees latitude and it's the beginning of summer, astronomical twilight never ends but bridges the night from dusk till dawn with its subtle presence in the northern sky. Keep going north and that subtle glow gets brighter and brighter until you arrive at 66 degrees north (the Arctic Circle) where the sun itself breaks over the horizon to shine throughout the entire night on the first day of summer. Never mind about night in the Arctic and Antarctic regions -- there's no such thing during the warmer months. It's all twilight and sunshine.
The difference in the angle the sun makes to the horizon changes the length of twilight. Although the same time -- one hour -- has passed, look how much farther the sun is below the horizon in Tucson vs. Duluth. Illustration: Bob King
Travel south of Duluth's latitude and twilights shorten. In Tucson and Miami twilight's only an hour and a half. The difference in twilight length is caused by the changing angle the sun makes to the horizon depending on your latitude. From May through July in Duluth the sun makes a shallow angle to the horizon as it rises and sets, so it takes a long time to drop out of sight. Further south, its path cuts the horizon at a steeper angle. Its quick disappearance makes for shorter twilights. Those who've traveled to tropical destinations are probably familiar with how fast the sky gets dark after sunset.
Let's travel to Miami and compare its night to Duluth's:
* Sunset Saturday -- 8 p.m.
* Evening twilight end -- 9:29 p.m.
* Morning twilight start -- 5:06 a.m.
* Sunrise -- 6:35 a.m.
Do the math and Miami has 7 1/2 hours of darkness or three hours more than Duluth. That just doesn't seem fair. We do have one little advantage over Miami here in Duluth though -- the sky's a lot darker when it's dark.
To find sunrise, sunrise and twilight length times for your location any day of the year, go to the U.S. Naval Observatory website and key in your location and the type of table you're interested in. Happy Venus watching!
"The grand show is eternal. It is always sunrise somewhere; the dew is never dried all at once; a shower is forever falling; vapor is ever rising. Eternal sunrise, eternal dawn and gloaming, on sea and continents and islands, each in its turn, as the round earth rolls." -- John Muir